We propose to develop F-18 and 1-123 labeled nucleosides as selective substrates of herpes simplex thymidine kinase (HSV1-TK) in conjunction with positron computed tomography (PET). Introduction of HSV1- TK in tumor followed by gancyclovir (GCV) treatment has been successfully tested as a suicide gene therapy for cancer therapy. Labeled nucleosides (such as [1-123/124]FIAU and [F-18JFHBG) selectively targeting the HSV1-TK enzyme are useful as reporter probes for measuring HSV1-tk gene expression in vivo by PET or SPECT imaging. Based on this approach, HSV1-tk gene expression can also be used as a surrogate marker gene for expression of other genes (transgenes) determined by the labeled nucleosides as in vivo imaging probes. The objective of this project is to design, synthesize and characterize a series of novel labeled nucleosides, 2'-deoxyuridine and FIAU analogs (group A and B), as superior imaging agents for measuring in vivo gene expression. Currently, [F-18]FIAU is more effective for imaging HSV1-TK; but the synthesis of this F-18 probe is very long and difficult. [F-18]FHBG is easier to prepare, but only effective for imaging a mutant HSV1-TK(sr39). The purposes of developing these novel nucleosides are: 1) to prepare F-18 radiolabeled group A and B nucleosides 2). to identify improved F-18 nucleoside probes with desired in vivo kinetics for higher target to non-target ratio and a higher selectivity between the human TK vs viral HSV1-TK enzyme and 3) to test the correlation between gene expression and probe-uptake in a doxycycline inducible HSV1-TK expressing cell line. It is likely that the new probes can be selectively phosphorylated by the viral enzyme, as such they may be superior for measuring the concentration of HSV1-TK enzyme levels in the tumor cells. They will have several useful properties: ability to cross the cell membrane and a minimum influence from other metabolic steps of nucleosides and nucleotides. Efforts will be made to select tracers predominantly trapped in the targeted cells by the native HSV1-TK only, and confounding signals from other processes from native hTK1 and hTK2 enzymes are minimized. In conjunction with PET, these new imaging agents may enhance our ability to measure tk gene expression in vivo; thus, they may enhance the probability of developing new gene therapy approaches for various diseases. [unreadable] [unreadable] [unreadable]